A PTP 650 can be powered using a + or - 48 VDC source when using the AC+DC Power Injector.
The PTP650 has an isolated supply in the ODU. The AC+DC Power Injector case is also isolated. So 48V supplies of either ground polarity (+48V or -48V) can be connected without fear of ground shorts.
The only caveat applies to PTP650 situations using the AUX port. If the AC+DC Power Injector is DC powered then the PoE out on the AUX port will also share the same ground as the DC supply. In general PoE equipment is isolated and so there should not be a risk of ground shorts.